Brief Overview of Human Nervous System

The human nervous system consists of two main parts, the central nervous system
(CNS) and the peripheral nervous system (PNS).
The CNS contains the brain and spinal cord. The PNS comprises the nerve fibers
that connect the CNS to every other part of the body. The PNS includes the motor neurons
that are responsible for mediating
voluntary movement. The PNS also includes the autonomic nervous system which
encompasses the sympathetic nervous system, the parasympathetic nervous system, and the
enteric nervous
system. The sympathetic and parasympathetic nervous systems are tasked with the
regulation of all involuntary activities. The enteric nervous system is unique
in that it represents a semi-independent part of the nervous system whose function is to control
processes specific to the gastrointestinal system. The nervous systems of the
body are composed of two primary types of cell: the neurons that carry the
chemical signals of nerve transmission, and the glial cells that serve to
support and protect the neurons.

Two important concepts relate to the
functioning of the nervous system. These terms are efferent and
afferent. Efferent connections in the nervous system refer to
those that send signals from the CNS to the effector cells of the body such as
muscles and glands. Efferent nerves are, therefore, also referred to as motor
neurons. Afferent connections refer to those that send signals from sense organs
to the CNS. For this reason these nerves are commonly referred to as sensory
neurons.

Another important cellular structure in nervous systems are the
ganglia. The term ganglion refers to a bundle
(mass) of nerve cell bodies. In the context of the nervous system, ganglia are
composed of soma (cell bodies) and dendritic structures. The dendritic trees of
most ganglia are interconnected to other dendritic trees resulting in the
formation of a plexus. In the human nervous system there are two main groups of
ganglia. The dorsal root ganglia, which is also referred to as the spinal
ganglia, contains the cell bodies of the sensory nerves. The autonomic ganglia
contain the cell bodies of the nerves of the autonomic nervous system. Nerves
that project from the CNS to autonomic ganglia are referred to as
preganglionic nerves (or fibers). Conversely, nerves projecting from ganglia to
effector organs are referred to as postganglionic nerves (or fibers).
Generally the term ganglion relates to the peripheral nervous system. However,
the term basal ganglia (also basal nuclei) is used commonly to describe the
neuroanatomical region of the brain that connects the hypothalamus, cerebral
cortex, and the brainstem.

Neurons

Neurons are the highly specialized cells of all nervous systems (e.g. CNS and
PNS) that are tasked with transmitting signals from one location to another.
These cells accomplish this role through specialized membrane-to-membrane
junctions called synapses. Most neuron possess an axon which is a long
protrusion from the body (soma) of the neuron to the synapse. Axons can extend
to distant parts of the body and make thousands of synaptic contacts such as is
the case with the CNS neurons of the spinal cord. Axons frequently travel
through the body in bundles called nerves. The synapses are termed pre-synaptic
and post-synaptic. The pre-synaptic synapse will release secretory granule
contents in response to the propagation of an electrochemical signal (action potential) down its
axon. The released substance (termed a neurotransmitter) will then, most likely,
bind to a specific receptor on the membrane of the post-synaptic synapse,
thereby, propagating the initial action potential to the next neuron. The human nervous
system is composed of hundreds of different types of neurons. These include
sensory neurons that transmute physical stimuli such as light and sound into
neural signals, and motor neurons that are responsible for converting neural signals into activation of muscles or glands.

Glial Cells

Glial cells (named from the Greek for "glue") are the specialized non-neuronal cells
of the nervous system that provide
protection, support and nutrition for neurons. As the Greek name glue infers, glial cells hold
neurons in place and provide guidance cues which directs axons of the neurons to
their appropriate target cell(s). Glial cells are responsible for the
maintenance of neural homeostasis, for the formation of myelin, and they play a
participatory role in signal transmission in the nervous system. Glial
cells provide an electrical insulation (myelin) for neurons which allows for
rapid transmission of action potentials and also prevents the abnormal
propagation of nerve impulses to inappropriate neurons. The glial cells that
produce the myelin sheath are called oligodendrocytes in the CNS and
Schwann cells in the PNS. Glial cells also destroy pathogens and remove dead
neurons.

Autonomic Nervous System

Sympathetic Nervous System

The sympathetic nervous system (SNS) is predominantly responsible for excitatory action potentials with the goal of inducing the "fight-or-flight" responses of the body under conditions of stress. In general, activation of the SNS results in contraction, for example, vasoconstriction. Although stress is a major trigger of the SNS, it is constantly active at a basal level to maintain homeostasis. The activation of the neurons of the SNS occurs as a result of signals arising in the region of the brain stem called the nucleus of the solitary tract (NTS, for the latin term nucleus tractus solitarii). The NTS receives a wide range of sensory inputs from both systemic and central baroreceptors and chemoreceptors. The neurons of the SNS emanate from the medulla, specifically the rostral ventrolateral medulla, and travel down the spinal cord where they synapse with short preganglionic neurons within the sympathetic ganglia. The ganglia of the SNS are the nerve cell bodies that lie on either side of the spinal cord. Preganglionic sympathetic fibers are those that exit the spinal cord synapses within these ganglia. The preganglionic neurotransmitter is acetylcholine, ACh. ACh released from the sympathetic preganglionic neuron binds to nicotinic ACh receptors (nAChR) on the postganglionic neuron. ACh binding depolarizes the cell body of the postganglionic neuron generating an action potential that travels to the target organ to elicit a response. The neurotransmitter released from sympathetic postganglionic neurons is norepinephrine which binds to its receptor expressed in the target cell. The target organ receptors responsive to signals from the SNS are those of the adrenergic family, specifically α1, α2, β1, and β2 (see below). Although the primary neurotransmitter released from sympathetic postganglionic neurons is norepinephrine, there are two important exceptions. These exceptions are the postganglionic neurons that innervate chromaffin cells of the adrenal medulla and those that innervate the sweat glands. When the postganglionic neurons that innervate sweat glands are activated, they release ACh (not epinephrine) which binds to muscarinic ACh receptors (mAChR: specifically the M1 and M3 receptors) on the target cell. Adrenal medullary chromaffin cells are functionally analogous to sympathetic postganglionic neurons and when stimulated by ACh from a sympthetic preganglionic neuron these cells release epinephrine and norepinephrine into the circulation. The receptors triggering the release of adrenal epinephrine and norepinephrine are nicotinic (nAChR).

Parasympathetic Nervous System

The parasympathetic nervous system is predominantly responsible for inhibitory action potentials resulting in relaxation, for example, vasodilation. The parasympathetic nervous system is responsible for stimulation of "rest-and-digest" and "feed-and-breed" activities that occur when the body is at rest. These responses include, but are not limited to, sexual arousal, salivation, lacrimation (tears), urination, digestion and defecation. Within the head the parasympathetic nervous system includes cranial nerves III, VII, and IX while cranial nerve X (comprising the vagus nerves) exits the brain stem to innervate the organs of the body. Like the SNS, the activation of the vagus nerves of the parasympathetic nervous system occurs as a result of signals arising in the NTS. There are three nuclei within the medulla that send out vagal nerves of the parasympathetic nervous system. These nuclei are the dorsal motor nucleus, the solitary nucleus, and the nucleus ambiguus. Parasympathetic neural outputs to the heart arise primarily within the nucleus ambiguus. The ganglia of the parasympathetic nervous system are also referred to as terminal ganglia as they lie close to, or within, the organs that they innervate. The exceptions to this are the parasympathetic ganglia of the head and neck. Parasympathetic ganglia are those that are found within the target organ. Preganglionic parasympathetic fibers associated with the vagal nerve all exit the brain stem, they do not travel down the spinal chord except for the pelvic splanchnic nerves which exit the spinal cord in the S2-S4 region. The parasympathetic preganglionic nerves enter their target organs where they form synapses with postganglionic neurons. Like the sympathetic ganglia, the neurotransmitter of parasympathetic preganglionic nerves is ACh. When released from these nerves the ACh binds to nicotinic ACh receptors (nAChR) on the postganglionic nerve. However, unlike sympathetic postganglionic nerves, activation of the parasympathetic postganglionic nerves results in the release of ACh. When released from the parasympathetic postganglionic neuron, the ACh binds to muscarinic ACh receptors (mAChR) in the target cells, primarily the M2 and M3 receptors.

Autonomic Control of Cardiovascular System

Within the cardiovascular system the norepinephrine released from sympathetic postganglionic neurons binds to β1 adrenergic receptors expressed on cells of the heart within the sinoatrial (SA) node (primary cardiac pacemaker cells), the atrioventricular (AV) node, the ventricles, and the conduction system. Activation of the β1 receptor in the heart results in increased force of contraction (inotropy), increased heart rate (chronotropy), and increased cardiac conductance (dromotropy). Within the vasculature sympathetic postganglionic nerve release of norepinephrine results in activation of the α1 and α2 adrenergic receptors resulting in vasoconstriction. However, the smooth muscle cells of the vessels in skeletal muscle possess predominantly β2 adrenergic receptors, stimulation of which results in vasodilation, since they need to remain open to receive the increased blood flow from the heart during the fight-or-flight response. The primary activator of the β2 adrenergic receptors in skeletal muscle vasculature is the epinephrine released from the adrenal medulla in response to sympathetic activation.

Within the cardiovascular system the primary target cells of the heart that receive parasympathetic innervation are the SA node (from the right vagus nerve), the AV node (from the left vagus nerve), and atrial cells. The cardiac muscarinic receptor that binds the ACh released from parasympathetic postganglionic nerves is the M2 type receptor. Each of the muscarinic ACh receptors is a GPCR and the M2 receptors are coupled to a Gi-type G-protein. Activation of the M2 receptor results in decreased levels of cAMP leading to reduced metabolic activity and also the activation of membrane K+ channels resulting in hyperpolarization of cardiac myocytes. The particular class of K+ channels that are responsive to G-proteins are activated by the βγ subunits of the G-protein. These K+ channels are commonly referred to as G protein-coupled inwardly-rectifying potassium channels (GIRK). The GIRK are members of the KCNJ subfamily of voltage-gated K+ channels. The net effect of M2 activation is decreased heart rate (chronotropy) and decreased cardiac conductance (dromotropy). The effects of the parasympathetic nervous system on the heart supercede the effects of the sympathetic nervous system such that even in the face of sympathetic stimulation, parasympathetic stimulation can depress cardiac activity. Within the vasculature ACh binds to the M3 receptor on endothelial cells leading to increased NO production resulting in vasodilation. However, this ACh is not derived from parasympathetic nerves but directly from the circulation. Parasympathetic postganglioninc ACh does stimulate M3 receptor-mediated NO production but this is only seen in the external genitalia.

Introduction to Neurotransmitters

Neurotransmitters are endogenous substances that act as chemical messengers by
transmitting signals from a neuron to a target cell across a synapse. Prior to their
release into the synaptic cleft, neurotransmitters are stored in secretory vesicles
(called synaptic vesicles) near the plasma membrane of the axon terminal.
The release of the neurotransmitter occurs most often in response to the arrival
of an action potential at the synapse. When released, the neurotransmitter
crosses the synaptic gap and binds to specific receptors in the membrane of the
post-synaptic neuron or cell.

Neurotransmitters are generally classified into two main categories related to
their overall activity, excitatory or inhibitory. Excitatory neurotransmitters
exert excitatory effects on the neuron, thereby,
increasing the likelihood that the neuron will fire an action potential. Major
excitatory neurotransmitters include glutamate, epinephrine and norepinephrine.
Inhibitory neurotransmitters exert inhibitory effects on the neuron, thereby,
decreasing the likelihood that the neuron will fire an action potential.
Major inhibitory neurotransmitters include GABA, glycine, and serotonin. Some neurotransmitters,
can exert both excitatory and inhibitory effects depending upon the type of receptors that are present.

In addition to excitation or inhibition, neurotransmitters can be broadly
categorized into two groups defined as small molecule neurotransmitters or
peptide neurotransmitters. Many peptides that exhibit neurotransmitter activity
also possess hormonal activity since some cells that produce the peptide secrete
it into the blood where it then can act on distant cells. Small molecule neurotransmitters include (but are not limited to)
acetylcholine, GABA, amino acid neurotransmitters, ATP and nitric oxide (NO).
The peptide neurotransmitters include more than 50 different peptides. Many of the gut-derived
and hypothalamic neurotransmitter peptides are discussed in detail in the
Gut-Brain Interrelationships page.
Several peptide neurotransmitters are all derived from the same precursor protein,
pro-opiomelanocortin (POMC), as discussed in the
Peptide Hormones page.

Many neurotransmitters can also be divided into two broad categories dependent upon
whether the receptor activated by the binding of transmitter is a metabotropic or an ionotropic
receptor. Metabotropic receptors activate signal transduction upon transmitter binding similar to
many peptide hormone receptors which involves a second messenger. Metabotropic receptors
are members of the G-protein coupled receptor (GPCR) family.
Ionotropic receptors ligand-gated ion channels. Some neurotransmitters, for example
glutamate and acetylcholine, bind to multiple receptors some of which are metabotropic and some of which are ionotropic.

Nerve Cell Action Potentials and Synaptic Transmission

The transmission of an efferent signal from the CNS to a target tissue, or an afferent signal from a peripheral tissue back to the CNS occurs as a result of the propagation of action potentials along a nerve cell. Nerve cells are excitable cells and they can respond to various stimuli such as electrical, chemical, or mechanical. When the excitation event is propagated along the nerve cell membrane it is referred to as a nerve impulse or more often as an action potential. When a nerve cell terminates on another it does so at a specialized structure called a synapse. Synaptic transmission refers to the propagation of nerve impulses (action potentials) from one nerve cell to another. The synapse is a junction at which the axon of the presynaptic neuron terminates at some location upon the postsynaptic neuron. The end of a presynaptic axon, where it is juxtaposed to the postsynaptic neuron, is enlarged and forms a structure known as the terminal button (pronounced "boo-tawn"). An axon can make contact anywhere along the second neuron: on the dendrites (an axodendritic synapse), the cell body (an axosomatic synapse) or the axons (an axo-axonal synapse).

Action potentials are the result of membrane depolarization which is brought about by a change in the distribution of ions across the membrane. Differences in ion concentrations on either side of a membrane result in a electrical charge differential across the membrane which is referred to as an electrochemical potential. Changes in ion concentrations on either side of a membrane result in depolarization of the membrane. Once a portion of a membrane is depolarized, the ion gradients need to be returned to the "resting" state, a process referred to as repolarization. The movement of ions across the membrane is the function of proteins and protein complexes termed ion channels. Because nerve transmission involves changes in voltage (charge) across the plasma membrane, these ion channels respond to the voltage changes and are, therefore, referred to as voltage-gated ion channels.

The resting membrane potential of a neuron is maintained by the differential distribution of K+ and Na+ ions. The concentration of intracellular K+ is much higher than the extracellular concentration. This situation is just the opposite for Na+, which is at a much higher concentration outside the cell than inside. This differential is maintained through the action plasma membrane transporters of the Na+,K+-ATPase family. The initiation and propagation of an action potential is the result of the opening and closing of voltage-gated K+ channels and voltage-gated Na+ channels. In the rested stated both types of voltage-gated channels are closed. In response to a depolarizing signal (an excitation signal) the fast acting voltage-gated Na+ channels open allowing an influx of Na+ ions into the cell. The influx of Na+ causes more voltage-gated Na+ channels to open propagating the depolarization event. The Na+ channels ultimately close (within milliseconds) to an inactivated state, meaning they cannot be re-opened prior to the membrane returning to its initial rested state. The opening of voltage-gated K+ channels occurs much slower than for the Na+ channels and they are not fully open until the Na+ channels have re-closed. The opening of the K+ channels allows K+ to exit the cell which brings the net charge inside the cell back to the rested state potential. The opening of the K+ channels, following closure of the Na+ channels, represents the repolarization stage and brings the action potential to an end.

Nerve impulses are transmitted from one neuron to another, or from a neuron to a target tissue cell, at synapses by the release of neurotransmitters. As discussed in detail throughout this page, neurotransmitters can be small chemicals, such as amino acids or amino acid derivatives, or they can be lipids, such as the endocannabinoid, anandamide. As a nerve impulse, or action potential, reaches the end of a presynaptic axon, molecules of neurotransmitter are released into the synaptic space. The release of neurotransmitter involves the processes of exocytosis. When an action potential reaches the presynaptic terminal the membrane depolarization results in the opening of voltage-gated Ca2+ channels. The influx of Ca2+ ions induces the membranes of neurotransmitter secretory vesicles to fuse with the plasma membrane allowing the contents to be released into the synaptic cleft.

Glutamate synapse. Structure of a typical synapse showing the presynaptic terminal and the postsynaptic terminal for a typical glutamatergic neural connection. This example depicts a synapse which involves glutamate activation of the three classes of ionotropic glutamate receptors. Definitions of the receptors types can be found in the section below discussing the glutamate-glutamine cycle in the brain.

Neuromuscular Transmission

In order to move a skeletal muscle cell, an action potential must be initiated from a peripheral motor neuron. Cardiac muscle (myocardial) cells on the other hand, can initiate their own electrical activity in the absence of an autonomic nerve-mediated action potential. With respect to skeletal muscle, nerve transmission occurs when an axon of a post-ganglionic nerve terminates on a skeletal muscle fiber, at specialized structures called the neuromuscular junction. An action potential occurring at this site is known as neuromuscular transmission. At a neuromuscular junction, the axon subdivides and branches into numerous structures, referred to as terminal buttons (pronounced "boo-tawns") or end bulbs, that can then innervate numerous skeletal muscle fibers. The result is that many muscle fibers can be innervated by a single neuron instead of each fiber having to be dependent upon an individual neuron for contractile activation. The skeletal muscle fibers that are innervated by branches from the same neuron constitute a motor unit. Large muscles in the body (e.g. the gastrocnemius) contain numerous motor units. This arrangment of the motor units in a particular muscle allows for activation of only a specific part of a muscle at any given time. This represents a form of spatial control over muscle fiber contraction within a muscle, a feature not associated with cardiac muscle excitation as discussed below.

The terminal buttons (end bulbs) of the motor neurons reside within depressions formed in the skeletal muscle plasma membrane (sarcolema). At these locations the skeletal muscle membrane is thickened and is referred to as the motor end plate. The space between the terminal buttons (end bulbs) and the motor end plate is similar to the synaptic cleft that exists where the pre-synaptic and post-synaptic membranes of neurons are in close proximity. The particular neurotransmitter in use at the neuromuscular junction is acetylcholine, ACh. When an action potential reaches the pre-synaptic membrane of a motor neuron the permeability of the membrane changes. This change in permeability allows Ca2+ to enter the nerve endings triggering exocytosis of ACh-containing vesicles. The released ACh then binds to nicotinic ACh receptors (nAChR) that are concentrated in the motor end plate membrane. Once released from the motor neuron, the level of active ACh is controlled by its catabolism through the action of acetylcholinesterase. As discussed below, nAChR are members of the ionotropic receptor superfamily (ion channel receptors). Activation of nicotinic ACh receptors in the motor end plate results in an increase in Na+ and K+ conductance through the nAChR channel. The resulting influx of Na+ into the skeletal muscle cell produces a depolarizing potential. As a result of this depolarization, action potentials are conducted in both directions, away from the motor end plate, along the muscle fiber. These action potentials are the result of the initial membrane depolarization and propagated across the surface membrane via the opening of voltage-gated Na+ channels. The action potential is then propagated down the T-tubule system which directly interacts with the sarcoplasmic reticulum, SR. Activation of the SR leads to the release, into the sarcoplasm (cytoplasm of muscle cells), of stored Ca2+ through the opening of Ca2+ release channels. The SR calcium release channels are also known as the ryanodine receptor (RYR) due to the fact that they were originally identified by their high affinity for the plant alkaloid ryanodine.The end result of the ACh-initiated propagating action potential is muscle contraction.

A particularly devastating disease that results from defects in the overall processes of neuromuscular nerve transmission is myasthenia gravis, MG. MG is a very serious disorder that is often times fatal. The characteristic features of the disease are weakened skeletal muscles that tire with very little exertion. MG is an auto-immune disease associated with antibodies to the nAChR of the neuromuscular junction. Binding of the antibodies to the receptor results in receptor destruction as well as receptor cross-linking. In most patients with MG there is a 70%–90% reduction in motor end plate nicotinic receptor number. Two major forms of MG exist, one in which the extraocular muscles are the ones primarily affected and in the other form there is a generalized skeletal muscle involvement. In the latter form of MG, the muscles of the diaphragm become affected resulting in respiratory failure which contributes to the mortality of MG. Treatment of MG involves numerous approaches including the use of acetylcholinesterase inhibitors. The use of these types of drugs allows for enhanced levels of ACh at the motor end plate during repeated muscle stimulation.

Neurotransmitter Receptors

Once the molecules of neurotransmitter are released
from a cell as the result of the firing of an action potential, they bind to specific
receptors on the surface of the postsynaptic cell. In all cases in which these
receptors have been cloned and characterized in detail, it has been shown that
there are numerous subtypes of receptor for any given neurotransmitter. As well
as being present on the surfaces of postsynaptic neurons, neurotransmitter
receptors are found on presynaptic neurons. In
general, presynaptic neuron receptors act to inhibit
further release of neurotransmitter.

The vast majority of neurotransmitter receptors belong
to a class of proteins known as the G-protein coupled receptors, GPCRs. Go to
the Signal Transduction page
for more information on theses receptors. The GPCRs are also called serpentine receptors
because they exhibit a characteristic transmembrane structure: that is, it
spans the cell membrane, not once but seven times. The link between
neurotransmitters and intracellular signaling is carried out by association
either with the receptor-associated G-protein, with
protein kinases, or by the receptor itself in the
form of a ligand-gated ion channel (for example, the
nicotinic acetylcholine receptors). The receptors that are of the GPCR family
are referred to as metabotropic receptors, whereas, the ligand-gated ion
channel receptors are referred to as ionotropic receptors.

One additional characteristic of neurotransmitter
receptors is that they are subject to
ligand-induced desensitization.
Receptor desensitization refers to the phenomenon whereby upon prolonged exposure ligand
results in uncoupling of the receptor from its signaling cascade. A common means
of receptor desensitization involves receptor phosphorylation by
receptor-specific kinases. Following phosphorylation of the receptor there is
increased affinity for inhibitory molecules that uncouple the interaction of
receptor with its associated G-protein. One major class of these desensitizing
inhibitors are the arrestins. Arrestins were first identified in studies of β-adrenergic
receptor desensitization and so were called β-arrestins.

Table of Common Neurotransmitters

non-inclusive listing

functions in both the CNS and the PNS; receptors are cholinergic;
2 receptor classes: muscarinic (metabotropic) and nicotinic (ionotropic); within the periphery
ACh is the major transmitter of the autonomic nervous system where it activates muscles; within the
brain its major effects are inhibitory or anti-excitatory; its actions in cardiac tissue
are also inhibitory

most abundant excitatory neurotransmitter in the CNS; glutamate binds to the metabotropic glutamate receptors
(mGluRs) of which there are eight (mGluR1–mGluR8) divided into three families;
glutamate also binds to several ionotropic receptors including the N-methyl-D-aspartate (NMDA) receptor
(NMDAR), the kainate receptors (KAR), and the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor
(AMPAR)

inhibitory neurotransmitter in the CNS primarily within the brainstem, spinal cord, and retina;
binds to glycine receptors (GlyR) which are ionotropic; there are two separate subunit proteins of each
GlyR (α and β) that combine in various ways to generate a pentameric structure;
there are four α-subunit genes (α1–4) and one β-subunit gene;
the primary adult form of GlyR is composed of three α1 subunits and two β subunits;
is also a required co-agonist with glutamate on NMDA receptors and in this capacity exerts an excitatory effect

produced by
mast cells, basophils, enterochromaffin-like cells (ECL) of the stomach, and hypothalamus; within the gut histamine
stimulates gastric parietal cells to secrete acid; released from mast cells when allergens bind to IgE-antibody complexes; there are four histamine receptors (H1–H4) all of which are GPCRs

most abundantly expressed in enterochromaffin cells of the gut where it regulates motility, also found in the CNS and platelets; released from activated platelets where it stimulates further activation propagating role of platelet aggregation in coagulation; in the CNS 5-HT regulates mood, appetite, sleep, memory and learning; selective serotonin re-uptake inhibitors (SSRIs) used in the treatment of depression

catecholamine neurotransmitter and hormone; binds to both α- and β-adrenergic receptors (GPCRs);
produced in the adrenal medulla and neurons in the CNS and PNS; primary hormone of the fight-or-flight response of the sympathetic nervous system; is a major regulator of metabolic processes in numerous tissues; regulates
heart rate, induces vasoconstriction and bronchodilation

catecholamine neurotransmitter and hormone; binds to both α- and β-adrenergic receptors (GPCRs);
produced in CNS and PNS by sympathetic nerves; major neurotransmitter function is in regulation of cardiac
inotropic (force) and chronotropic (rate) activities; functions along with epinephrine in the fight-or-flight response; involved in adaptive thermogenesis in brown adipose tissue (BAT)

within the CNS dopamine plays a major role in reward-motivated behavior such as feeding and drug-seeking behaviors;
also involved in motor control; in the periphery dopamine regulates the release of several hormones such as insulin
from the pancreas and norepinephrine from blood vessels; functions by binding to a family of dopaminergic receptors (GPCRs)

an endocannabinoid, binds to the cannabinoid receptors (CB1 and CB2)
with highest affinity for CB1; CB1 is most abundant receptor in the
CNS; classic response to CB1 activation is stimulation of food intake; exerts
peripheral effects on overall energy homeostasis

Adenosine

other

ATP

is an inhibitory neurotransmitter within the CNS, suppresses arousal thus promoting sleep;
within the periphery adenosine exerts anti-inflammatory actions, stimulates vasodilation through vascular smooth muscle, induces bronchospasm in the lungs; within the heart it affects the cardiac conduction system; adenosine binds to a family of adenosine receptors (members of GPCR family) identified as A1 (coupled to Gi/o), A2a (coupled to Gs), A2b (coupled to Gs or Gq dependent upon tissue), and A3 (coupled to Gq); activation of A1 receptors expressed in cardiac pacemaker cells of sinoatrial node leads to reduced heart rate (chronotropy); activation of A2a receptors in coronary vascular smooth muscle cells induces vasodilation;

ATP

other

as a neurotransmitter ATP is released from sympathetic, sensory and enteric nerves; ATP binds to metabotropic G-protein coupled receptors (GPCR) of the P2Y family of purinergic/pyrimidinergic receptors of which there are ten in humans: P2Y1, 2, 4, 6, 8, 10–14; P2Y12 is primarily expressed on the surface of platelets where it serves as a major ATP-mediating receptor of blood coagulation

ATP also binds to the ionotropic family of purinergic receptors (P2X) which consists of seven members (P2X1-P2X7); these receptors modulate synaptic transmission throughout the autonomic nervous systems of the CNS and PNS; in the periphery the P2X receptors activate contractile activity of various muscle types

Glutamate: Major Excitatory Neurotransmitter

Within the CNS glutamate is the main excitatory neurotransmitter. Neurons that respond to glutamate
are referred to as glutamatergic neurons. Postsynaptic glutamatergic neurons possess three distinct types of
ionotropic receptors that bind glutamate released from presynaptic neurons. These ionotropic receptors have been
identified on the basis of their binding affinities for certain substrates and are, thus referred to as the the kainate,
2-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate
(NMDA) receptors, and the delta (δ) receptors. Each of these classes of glutamate
receptor subunit form ligand-gated ion channels, thus the derivation of the term ionotropic. There are
also multiple subtypes of each of these classes of ionotropic glutamate receptor subunits.

The AMPA receptor subunits are referred to as GluA1 (GluR1) through GluA4 (GluR4) and each is encoded by separate genes.
Functional AMPA receptors consist of heterotetramers that are formed from dimers of GluA2 and dimers of either GluA1, GluA3, or GluA4.
The GluA2 subunit of the receptor is responsible for regulating the permeability of the channel to calcium ions.
The GluA2 mRNA is subject to RNA editing which alters the function of the calcium permeability character of the subunit.
For details on the editing of the GluA2 mRNA go to the RNA Metabolism page.
The AMPA receptors are found on most excitatory postsynaptic neurons where they mediate fast excitation.
Indeed, AMPA receptors are responsible for
the bulk of fast excitatory synaptic transmission throughout the CNS. The concept of fast synaptic transmission relates to the
fact that the ion channel opens and closes quickly in response to ligand (e.g. glutamate) binding. The ion permeability
of the AMPA receptors is controlled by the GluA2 subunit. AMPA receptors have low permeability to calcium ions even in the
ligand-activated state and this is to prevent excitotoxicity in these neurons.

The NMDA receptor is generated from two separate subunit families. These subunit families are identified as GluN1 (also called NMDAR1)
and GluN2. There are four GluN2 subunits (GluN2A–GluN2D; also NMDAR2A–NMDAR2D).
The four different GluN2 subunits are encoded by distinct genes. Although there is a single gene encoding the GluN1 subunit, multiple
isoforms of this subunit are generated through alternative splicing events. The functional NMDA receptor is composed of a heterotetramer with all
forms containing the GluN1 subunit and one of the different GluN2 subunits. Unlike the other ionotropic glutamate receptors, the NMDA receptors
are activated by simultaneous binding of glutamate and glycine. Glycine serves as a co-agonist and both amino acid neurotransmitters
must bind in order for the receptor to be activated. Glycine binds to the GluN1 subunit while glutamate binds to the GluN2 subunit.
Glutamate binding to NMDA receptors results in calcium influx into the postsynaptic cells leading to the activation
of a number of signaling cascades. These signaling cascades can include activation of calcium/calmodulin-dependent kinase II (CaMKII)
leading to phosphorylation of the GluA2 AMPA receptor subunit. This latter effect results in long-term potentiation (LTP).
NMDA receptor activation also triggers PKC-dependent insertion of AMPA receptors into the synaptic membrane during LTP as well as activation
of the kinases PI3K, AKT/PKB, and GSK3, each of which modulates LTP.

The kainate receptor subunits are known as GluK1 through GluK5 (formerly GluR5, GluR6, GluR7, KA1, and KA2).
The GluK1–GluK3 subunits can form hetero- and homomeric receptor complexes. In addition, alternative splicing of the
GluK1 and GluK2 mRNAs results in at least five distinct subtypes (GluK1a–GluK1c, GluK2a, GluK2b).
Less is known about the physiological significance of the kainate receptors. One major role of the kainate receptors
is in the regulation of synaptic plasticity. Another important function of the kainate receptors is in the regulation of the release of the
inhibitory neurotransmitter GABA. This function of the kainate receptors is due to their
presence on presynaptic GABAergic neurons.

The delta (δ) glutamate receptors were identified as ionotropic glutamate receptors based upon amino acid
sequence similarity to the other more well-characterized ionotropic glutamate receptors. However, these proteins
do not form glutamate-gated functional ion channels either alone or in combination with any of the other ionotropic
glutamate receptor proteins. Indeed, these proteins do not bind glutamate or any other excitatory amino acid receptor
ligands. The GluD1 receptor (encoded by the GRID1 gene) is prominently expressed in inner ear hair cells and neurons of the hippocampus.
The presentation of GluD1 in the inner ear indicates that it has a role in hearing. The GluD2 receptor (endcoded
by the GRID2 gene) is expressed exclusively in the Purkinje cells of the cerebellum. GluD2 function is critical for the
development of neuronal circuits and functions that includes long-term depression (LTD), learning and memory.

Within the CNS glutamatergic neurons are
responsible for the mediation of many vital processes such as the encoding of information, the formation and retrieval of memories,
spatial recognition and the maintenance of consciousness. Excessive excitation of glutamate receptors
has been associated with the pathophysiology of hypoxic injury, hypoglycemia, stroke and epilepsy.

Glutamate can also bind to another class of receptor termed the metabotropic glutamate receptors
(mGluRs; where the small m refers to metabotropic). There are eight
known metabotropic glutamate receptors identified as mGluR1–mGluR8. Unlike the ionotropic receptors, the mGluRs
are members of the G-protein coupled receptor (GPCR) family. The mGluRs can be divided into three distinct subclasses based upon
sequence similarities and receptor associated G-protein. Group I mGluRs include mGluR1 and mGluR5, both of which are coupled to Gq
type G-proteins and upon activation trigger increased production of DAG and IP3. Group II is composed of mGluR2 and mGluR3.
Group III is composed of mGluR4, mGluR6, mGluR7, and mGluR8. Both group II and III mGluRs activate an associated Gi
type G-protein resulting in decreased production of cAMP. The mGluRs are primarily expressed on neurons and glial cells in
close proximity to the synaptic cleft. Within the CNS, mGluRs modulate the neurotransmitter effects of glutamate as well
as a variety of other neurotransmitters. In addition to the CNS, mGluRs have a widespread distribution in the periphery.
Given their wide pattern of expression, diverse roles for mGluRs have been suggested. Some of these processes include control of
hormone production in the adrenal gland and pancreas, regulation of mineralization in the developing cartilage,
modulation of cytokine production by lymphocytes, directing the state of differentiation in embryonic stem cells,
and modulation of secretory functions within the gastrointestinal tract.

GPCR coupled to Gi-type G-protein; primarily a presynaptic
receptor; involved in a photoreceptor-independent form of light adaptation within the retina;
found on the photoreceptor–On bipolar cell synapse

mGluR7

GRM7

metabotropic, group III family

GPCR coupled to Gi-type G-protein; most widely distributed pre-synaptic mGluR;
found at a wide range of synapses postulated to be critical for both normal CNS function
and several human disorders; is a key regulator in shaping synaptic responses at glutamatergic synapses
as well as in regulating critical aspects of inhibitory GABAergic transmission

mGluR8

GRM8

metabotropic, group III family

GPCR coupled to Gi-type G-protein; primarily a pre-synaptic
receptor; involved in anxiety by depressing excitatory synaptic transmission
in the bed nucleus of the stria terminalis (BNST)

GluA1 (GluR1)

GRIA1

ionotropic: AMPA

responsible for the bulk of fast excitatory synaptic transmission throughout the CNS

GluA2 (GluR2)

GRIA2

ionotropic, AMPA

controls the Ca2+ permeability of the AMPA receptor channels;
RNA editing controls
the permeability by altering a single amino acid (the Q/R site) in the second
transmembrane domain (TMII) of the protein, if unedited the Q residue allows
Ca2+ permeability whereas the edited amino acid (R) does not;
almost all the CNS GluA2 is edited

GluA3 (GluR3)

GRIA3

ionotropic, AMPA

responsible for the bulk of fast excitatory synaptic transmission throughout the CNS

GluA4 (GluR4)

GRIA4

ionotropic, AMPA

responsible for the bulk of fast excitatory synaptic transmission throughout the CNS

GluK1 (GluR5)

GRIK1

ionotropic, Kainate

three splice variants

GluK2 (GluR6)

GRIK2

ionotropic, Kainate

two splice variants

GluK3 (GluR7)

GRIK3

ionotropic, Kainate

GluK4 (KA1)

GRIK4

ionotropic, Kainate

expressed almost exclusively in the hippocampus

GluK5 (KA2)

GRIK5

ionotropic, Kainate

protein retained within the ER unless assembled into a complex with either GluK1, GluK2, or GluK3

GluN1 (NR1, NMDAR1)

GRIN1

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; GluN1
provides the glycine-binding site as does the GluN3 subunits; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN2A (NR2A, NMDAR2A)

GRIN2A

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; the GluN2
subunits provide the glutamate-binding sites; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN2B (NR2B, NMDAR2B)

GRIN2B

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; the GluN2
subunits provide the glutamate-binding sites; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN2C (NR2C, NMDAR2C)

GRIN2C

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; the GluN2
subunits provide the glutamate-binding sites; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN2D (NR2D, NMDAR2D)

GRIN2D

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; the GluN2
subunits provide the glutamate-binding sites; receptors function asmodulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN3A (NR3A, NMDAR3A)

GRIN3A

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; GluN3
subunits provide the glycine-binding sites as does the GluN1 subunit; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

GluN3B (NR3B, NMDAR3B)

GRIN3B

ionotropic, NMDA

functional NMDA receptors requires simultaneous binding of both glutamate and glycine; GluN3
subunits provide the glycine-binding sites as does the GluN1 subunit; receptors function as modulators of
synaptic response and are involved in co-incidence detection (bidirectional current flow at a synapse)

The Glutamate-Glutamine Cycle in the Brain

Within the CNS there is an interaction between the cerebral blood flow, neurons, and the protective astrocytes
that regulates the metabolism of glutamate, glutamine, and ammonia. This process is referred to as the
glutamate-glutamine cycle
and it is a critical metabolic process central to overall brain glutamate metabolism. Using presynaptic neurons
as the starting point, the cycle begins with the release of glutamate from presynaptic secretory vesicles in response
to the propagation of a nerve impulse along the axon. The release of glutamate is a Ca2+-dependent process
that involves fusion of glutamate containing presynaptic vesicles with the neuronal membrane.
Following release of the glutamate into the synapse it must be rapidly removed to prevent over excitation of the
postsynaptic neurons. Synaptic glutamate is removed by three distinct process. It can be taken up into the postsynaptic
cell, it can undergo reuptake into the presynaptic cell from which it was released or it can be taken up
by a third non-neuronal cell, namely astrocytes. Postsynaptic neurons remove little glutamate from the synapse and although there
is active reuptake into presynaptic neurons the latter process is less important than transport into astrocytes.
The membrane potential of astrocytes is much lower than that of neuronal membranes and this favors the uptake of
glutamate by the astrocyte. Glutamate uptake by astrocytes is mediated by Na+-independent and Na+-dependent systems.
The Na+-dependent systems have high affinity for glutamate and are the predominant
glutamate uptake mechanism in the central nervous system. There are two distinct astrocytic Na+-dependent
glutamate transporters identified as EAAT1 (for Excitatory
Amino Acid Transporter 1; also called GLAST) and EAAT2 (also called GLT-1).

Brain glutamate-glutamine cycle. Ammonium ion (NH4+) in the blood is taken up by astrocytes and incorporated
into glutamate via glutamine synthetase. The glutamine then is transported to presynaptic neurons via SLC38A7 (also called
sodium-coupled neutral amino acid transporter 7, SNAT7). Within the presynaptic neuron glutamate is formed from the glutamine via
the action of glutaminase. The glutamate is packaged in secretory vesicles for release following activation of an action potential.
Glutamate in the synaptic cleft can be taken up by astrocytes via the EAAT1 and EAAT2 transporters (excitatory amino acid transporters
1 and 2; also known as glial high affinity glutamte transporters). Within the astrocyte the glutamate is converted back to glutamine.
Some of the astrocyte glutamine can be transported into the blood via the action of the transporter SLC38A3 (also called
sodium-coupled neutral amino acid transporter 3, SNAT3).

Following uptake of glutamate, astrocytes have the ability to dispose of the amino acid via export into the
blood though capillaries that contact the foot processes of the astrocytes. The problem with glutamate disposal via
this mechanism is that it would eventually result in a net loss of carbon and
nitrogen from the CNS. In fact, the outcome of astrocytic glutamate uptake is its conversion to glutamine.
Glutamine thus serves as a "reservoir" for glutamate but in the form of a non-neuroactive
compound. Release of glutamine from astrocytes allows neurons to derive glutamate from this parent compound.
Astrocytes readily convert glutamate to glutamine via the glutamine synthetase catalyzed reaction as this microsomal
enzyme is abundant in these cells. Indeed, histochemical data demonstrate that the glia
are essentially the only cells of the CNS that carry out the glutamine synthetase reaction. The ammonia that is used to
generate glutamine is derived from either the blood or from metabolic processes occurring in the brain.

Like the uptake of glutamate by astrocytes, neuronal glutamine uptake proceeds via
both Na+-dependent and Na+-independent mechanisms. The major glutamine transporter
in both excitatory and inhibitory neurons is the system N neutral amino acid transporter SLC38A7 (also called SNAT7).
The predominant metabolic fate of the glutamine taken up by neurons is hydrolysis to glutamate and
ammonia via the action of the mitochondrial form of glutaminase
encoded by the GLS2 gene. This form of glutaminase is referred to as phosphate-dependent glutaminase (PAG). The inorganic phosphate (Pi)
necessary for this reaction is primarily derived from the hydrolysis of ATP and its function is to lower
the KM of the enzyme for glutamine. During depolarization there is a sudden increase in
energy consumption. The hydrolysis of ATP to ADP and Pi thus favors the concomitant
hydrolysis of glutamine to glutamate via the resulting increased Pi. Because there
is a need to replenish the ATP lost during neuronal depolarization, metabolic
reactions that generate ATP must increase. It has been found that not all neuronal glutamate derived from
glutamine is utilized to replenish the neurotransmitter pool. A portion of the glutamate can be oxidized within the nerve cells
following transamination. The principle transamination reaction involves aspartate aminotransferase (AST) and yields
α-ketoglutarate (2-oxoglutarate) which is a substrate in the TCA cycle. Glutamine, therefore,
is not simply a precursor to neuronal glutamate but a potential fuel, which, like glucose, supports neuronal energy
requirements.

Glutamate, released as a neurotransmitter, is taken up by astrocytes, converted to glutamine, released back to
neurons where it is then converted back to glutamate represents the complete glutamate-glutamine cycle.
The significance of this cycle to brain glutamate handling is
that it promotes several critical processes of CNS function. Glutamate is rapidly removed from
the synapse by astrocytic uptake thereby preventing over-excitation of the postsynaptic neuron. Within
the astrocyte glutamate is converted to glutamine which is, in effect, a non-neuroactive compound that can be transported
back to the neurons. The uptake of glutamine by neurons provides a mechanism for the regeneration of glutamate which
is augmented by the generation of Pi as a result of ATP consumption during depolarization. Since
the neurons also need to regenerate the lost ATP, the glutamate can serve as a carbon skeleton for oxidation in the TCA cycle.
Lastly, but significantly, the incorporation of ammonia into glutamate in the astrocyte serves as a mechanism to buffer
brain ammonia.

Glycine

Glycine, as an amino acid found in proteins, is critical to the functions of several different classes of protein, particularly those of the
extracellular matrix. However, glycine as a free amino acid also functions as a highly important neurotransmitter within the central nervous system, CNS. Glycine and GABA are the major inhibitory neurotransmitters in the CNS, whereas, glutamate is the major excitatory neurotransmitter. In conjunction with glutamate, glycine can also function in an excitatory capacity as a co-agonist acting on the NMDA subtype of glutamate receptors (see section above). The receptors to which glycine binds were originally identifed by their sensitivity to the alkaloid strychnine. Strychnine-sensitive glycine receptors (GlyRs) mediate the synaptic
inhibition exerted in response to glycine binding. Glycinergic synapses mediate fast inhibitory neurotransmission within the spinal cord, brainstem, and caudal brain. The effects of glycine exert control over a variety of motor and sensory functions, including vision and audition. The GlyRs are members of the ionotropic family of ligand-gated ion channels. The binding of glycine leads to the opening of the GlyR integral anion channel, and the resulting influx of Cl– ions hyperpolarizes the postsynaptic cell, thereby
inhibiting neuronal firing.

Glycine Transporters

Cellular uptake of glycine, particularly within neurons in the central nervous system (CNS), is regulated by the presence of specific glycine transporters identified as GlyT. There are two subtypes of GlyT identified as GlyT1 and GlyT2. Both glycine transporters are members of the
solute carrier family of membrane transporters. The GlyT1 protein is encoded by the SLC6A9 gene and the GlyT2 protein is encoded by the SLC6A5 gene. The tissue distribution and funciton of the two glycine transporters are distinct. GlyT1 is predominantly expressed in glutamatergic neurons where it functions in the regulation of glycine levels in the vicinity of the NMDA-type
glutamate receptors. GlyT2 is predominantly expressed in glycinergic neurons where it functions to regulate
inhibitory glycinergic neurotransmission by decreasing synaptic Gly concentrations after
presynaptic release. A form of inherited hyperekplexia of presynaptic origin (HKPX3) results from mutations in the SLC6A5 (GlyT2) gene.

Impaired glutamatergic neurotransmission via the NMDA receptors has been associated with the symptoms of schizophrenia and the associated cognitive deficit. Pharmacologic inhibitors of GlyT1 have some utility to improve impaired NMDA receptor function in psychosis by increasing
synaptic glycine concentrations. These transport inhibitors function by increasing extrasynaptic Gly concentrations via inhibition of its neuronal or glial reuptake
processes. When used in combination with other antipsychotic medications, GlyT1 inhibitors have been shown to be capable of restoring disturbed glutamatergic-GABAergic-dopaminergic
balance in psychosis.

Glycine Receptors

The receptors to which glycine binds (GlyRs) are members of the group I ligand-gated ion channel (LGIC) class of receptors. The LGIC receptors are members of the Cys loop receptor family that also includes the nicotinic acetylcholine receptors (nAChR), the serotonin type 3 receptor (5-HT3), and the
GABAA receptors (GABAAR). The GlyRs are composed of three different proteins, two of which constitute the actual receptor and a third protein that serves a scaffolding function. The receptor subunits are referred to as GlyRα and GlyRβ. These subunits are tightly bound to a cytosolic scaffolding protein identified as gephyrin. Gephyrin is tightly bound to the GlyRβ subunit. In addition to its role in GlyR function, gephryin (gene symbol: GPHN) functions to regulate the activity of the GABAA receptor and it is required for molybdenum cofactor biosynthesis. Functional GlyRs are heteropentameric proteins similar to the organization of the nAChRs found in skeletal
muscle. The typical subunit composition of the heteropentameric GlyR is (GlyRα)2(GlyRβ)3.

Humans express four GlyR genes encoding α subunits (GLRA1–GLRA4) and a single GlyR gene encoding the β subunit (GLRB). All GlyRα subunits display high amino acid sequence identity and form functional homomeric
glycine-gated channels. The GlyRα subunits possess critical
determinants of ligand binding. The GLRA1 gene is located on chromosome 5q32 and is composed of 10 exons that generate three alternatively spliced mRNAs. The GLRA2 gene is located on the X chromosome (Xp22.2) and is composed of 13 exons that generate four alternatively spliced mRNAs. Two of the splice variant GLRA2 mRNAs encode the same protein, thus, the four variant mRNAs generate three different GLRA2 proteins. The GLRA3 gene is located on chromosome 4q34.1 and is composed of 13 exons that generate two alternatively spliced mRNAs. The GLRA4 gene is located on the X chromosome (Xq22.2) on the other arm relative to the position of the GLRA2 gene. The GLRA4 gene is composed of 9 exons that generate two alternatively spliced mRNAs. Glycine receptors that contain the GlyRα1 subunit represent the predominant form of the α-subunit in adult glycine receptors. Several mutations in the GLRA1 gene have been shown to be associated with the startle disease known as hereditary hyperekplexia type 1, HKPX1. The hallmark symptoms of HKPX1 are an exaggerated startle response to auditory
or tactile stimuli and, particularly in neonates, transient muscle
rigidity referred to as “stiff baby syndrome".

In addition to alternative splicing, the GLRA3 mRNA is subject to editing that results in the substitution of a Pro residue for a Leu residue at amino acid 185 in the extracellular domain. This version of the GlyRα3 protein confers an increased agonist affinity to GlyRα3-containing glycine receptors. The GlyRα3-containing GlyRs are involved in the pathways of nociception (pain sensation) within the spinal cord. Specific spinal cord neurons (in laminae I and II) mediate pain sensation in response to the inflammatory mediator, prostaglandin E2 (PGE2). When PGE2 binds to its receptor in these neurons (the EP2 receptor), PKA is activated which then phosphorylates the GlyRα3 protein in the glycine receptor resulting in down-regulation of glycine stimulated inhibitory circuits in these neurons. The analgesic effects of cannabinoids and
endocannabinoids involves the modulation of GlyRα3-containing glycine receptors. Thus, it is postulated that GlyRα3 represents a potentially useful target for the pharmacologic intervention in chronic pain syndromes.

The GlyRβ gene is located on chromosome 4q31.3 and is composed of 12 exons that generate three alternatively spliced mRNAs that encode two distinct GlyRβ isoforms. Unlike the GlyRα subunits which can form a functional glycine-gated ion channel, the GlyRβ protein cannot form a functional glycine receptor on its own. The role of the GlyRβ subunit is to regulate agonist binding and intracellular trafficking and synaptic clustering of post-synaptic GlyRs. Mutations in the GLRB gene are associated with another form of hyperekplexia identified as HKPX2.

GABA (γ-Aminobutyric acid)

Several amino acids have distinct excitatory or
inhibitory effects upon the nervous system. The amino acid derivative, γ-aminobutyrate (GABA; also called
4-aminobutyrate) is a major inhibitor of presynaptic
transmission in the CNS, and also in the retina. Neurons that
secrete GABA are termed GABAergic.

GABA cannot cross the blood-brain-barrier and as such must be synthesized within neurons in the CNS. The
synthesis of GABA in the brain occurs via a metabolic pathway referred to as the GABA shunt. Glucose is the
principal precursor for GABA production via its conversion to α-ketoglutarate in the TCA cycle.
Within the context of the GABA shunt the α-ketoglutarate is transaminated to glutamate by GABA α-oxoglutarate
transaminase (GABA-T). Glutamic acid decarboxylase (GAD) catalyzes the decarboxylation of
glutamic acid to form GABA. There are two GAD genes in humans identified as GAD1 and GAD2.
The GAD isoforms produced by these two genes are identified as GAD67 (GAD1 gene: GAD67) and GAD65
(GAD2 gene: GAD65) which is reflective of their molecular weights. Both the GAD1 and GAD2 genes are
expressed in the brain and GAD2 expression also occurs in the pancreas. The activity of GAD requires
pyridoxal phosphate (PLP) as a cofactor.
PLP is generated from the B6 vitamins (pyridoxine, pyridoxal, and pyridoxamine)
through the action of pyridoxal kinase. Pyridoxal kinase itself requires zinc
for activation. A deficiency in zinc or defects in pyridoxal kinase can lead to
seizure disorders, particularly in seizure-prone pre-eclamptic patients
(hypertensive condition in late pregnancy). The presence of anti-GAD antibodies (both anti-GAD65 and anti-GAD67) is a
strong predictor of the future development of type 1 diabetes
in high-risk populations.

GABA synthesis: The synthesis of GABA is a single step reaction involving the decarboxylation glutamate being catalyzed by glutamate decarboxylases (GAD).

GABA exerts its effects by binding to two distinct
receptor subtypes. The GABA-A (GABAA) receptors are members of the ionotropic receptors, specifically the Cys-loop subfamily of ligand-gated ion channels that includes the nicotinic
ACh receptors (nAChR), glycine receptors (GlyR), and the 5-HT3 (serotonin) receptor. The GABA-B (GABAB) receptors belong to the class C family of metabotropic G-protein coupled receptors (GPCR).
The GABA-A receptors are members of the ionotropic receptor family and are chloride channels that, in response to GABA binding,
increase chloride influx into the GABAergic neuron. The GABA-B receptors are coupled to a G-protein that activates an associated potassium
channel that when activated by GABA leads to potassium efflux from the cell.
The anxiolytic drugs of the benzodiazepine family exert their soothing effects by potentiating
the responses of GABA-A receptors to GABA binding.

Functional GABA-A receptors are generated by the combination of a wide array of different subunits.
A total of 19 GABA-A receptor subunit genes have been identified in humans that code for α (alpha),
β (beta), γ (gamma), δ (delta), ε (epsilon), π (pi), θ (theta), and ρ (rho).
The overall diversity of GABA-A receptors is further increased as several of theses genes undergo alternative
splicing. The complexity of the diverse array of molecular compositions of the GABA-A receptors has important functional
and clinical consequences as they determine the properties and pharmacological modulations of a given receptor
complex. In addition, zinc ions are known to regulate GABA-A receptor activity via inhibition of the receptor
through an allosteric mechanism that is critically dependent on the receptor subunit composition.
The GABRG3 (γ3 subunit gene) encoded protein is critical to this zinc-mediated regulation.
Although the minimal requirement to produce a functional GABA-gated ion channel is the
inclusion of both α and β subunits, the most common type in the brain is a heteropentameric
complex composed of two α subunits, two β subunits, and a γ subunit (α2β2γ).
The GABA-A receptors bind two molecules of GABA and in the heteropentameric receptors
this binding site is created by the interface between the α and β subunits.

The GABA-Aρ subunits do not form heteromeric complexes with other GABA-A receptor subunits but only
form homomeric receptor complexes. The GABA-Aρ receptors were formerly referred to as the GABA-C receptors.

The anxiolytic/sedative effects of the barbiturates and benzodiazepines are exerted via their
binding to subunits of the GABA-A receptors. Benzodiazepines bind to a site on the GABA-A receptor
created by the association of the gamma (γ) subunit and one of the the alpha (α) subunits. There
are two distinct subtypes of benzodiazepine receptors termed BZ1 (BZ1) and BZ2
(BZ2). The BZ1 receptor is formed by the interaction of γ and α1 subunits, whereas the
BZ2 receptors is formed by the interaction of the γ and α2, α3 or
α5 subunits. The receptor for the barbiturates is the beta (β) subunit of the GABA-A
receptor. When benzodiazepines bind to
the GABA-A receptor they potentiate the actions of GABA and require the presence of GABA in order for
activation of the ion channel. Barbiturates can induce GABA-A channel opening in the absence of GABA
when administered at high dose and as a result they can be lethal due to the level of CNS suppression.
The potential for lethal toxicity of a benzodiazepine requires an extremely large dose. This difference in toxicity between
barbiturates and benzodiazepines is the major reason barbiturates are not often used clinically any longer.

Under physiological conditions the binding of GABA to any of the GABA-A receptors leads to membrane hyperpolarization
and a reduction of action potential firing. However, studies have also demonstrated the GABA-A activation can
result in membrane reversal potential that is close to, or even at a more depolarized potential than the resting
membrane potential at a synapse. This results in a membrane depolarization referred to as shunting inhibition.
Shunting inhibition is also called divisive inhibition and defines a form of post-synaptic potential inhibition.
The term shunting is used because the synaptic conductance short-circuits currents that are generated
at adjacent excitatory synapses. If a shunting inhibitory synapse is activated, the amplitude of subsequent
excitatory postsynaptic potentials (EPSPs) is reduced. The major effect of GABA-A receptor activation is
reduced dendritic excitatory glutamatergic responses as a consequence of a local increase in conductance
across the plasma membrane. In addition to shunting inhibition, the polarity of GABA-A receptor-mediated
responses can change during different physiological or pathological conditions. For example, GABA triggers
excitation during the day and inhibition during the night within neural circuits of the suprachiasmatic nucleus.
Also, the repeated activation of GABA-A receptors can lead to a switch from a hyperpolarizing to
depolarizing direction and can, thus, enhance cell firing. The activation of GABA-A receptors results in both
phasic inhibitory postsynaptic currents (IPSCs) and tonic currents. The GABA-A-induced tonic
current result from GABA acting on extrasynaptic receptors composed of a different subunit
composition and therefore, different pharmacological activity compared with the synaptic receptors.

GABA-A Receptor Subunits

Receptor Subunit

Gene Symbol

Functions / Comments

GABA-A alpha 1 (α1)

GABRA1

GABRA1 protein is phosphorylated in a glycolysis-dependent reaction involving
a kinase activity associated with the enzyme glyceraldehyde 3-phosphate
dehydrogenase (GAPDH),
GAPDH-mediated phosphorylation maintains functionality of the protein; this process implicates a link between
regional cerebral glucose metabolism and GABAergic currents since the mechanism depends on
locally produced glycolytic ATP and GAPDH activity; cortical tissue isolated from epileptic patients
contains GABRA1 subunits in a reduced phosphorylation state compared to tissue from non-epileptic
individuals; mutations in the GABRA1 gene associated with susceptibility to juvenile myoclonic epilepsy

GABA-A alpha 2 (α2)

GABRA2

polymorphisms in the GABRA2 gene associated with susceptibility to alcohol dependence;
pharmacologic-specific activation of the GABA-A α2 subunit is highly effective against inflammatory
and neuropathic pain without sedation typical of benzodiazepine activation of the α1 subunit

GABA-A alpha 3 (α3)

GABRA3

similar to effects at the α2 subunit, pharmacologic-specific activation of the GABA-A α3
subunit is highly effective against inflammatory and neuropathic pain without sedation
typical of benzodiazepine activation of the α1 subunit

variable numbers of a partial duplication in the GABRA5 gene are found in different individuals; the GABRA5
gene is located within the chromosome 15 imprinted region found deleted in
Prader-Willi and Angelman syndromes;
the duplication number is higher in individuals with cytogenetically detectable deletions in the 15q region

GABA-A alpha 6 (α6)

GABRA6

cerebellar motor control is likely to be a distinct behavioral function associated
with GABA-A receptors that contain the α6 subunit; disruption in expression of
the GABRA6 gene leads to an associated loss of expression from the GABRD gene

GABA-A beta 1 (β1)

GABRB1

GABA-A beta 2 (β2)

GABRB2

GABA-A beta 3 (β3)

GABRB3

the GABRA3 gene is located within the chromosome 15 imprinted region found deleted in
Prader-Willi and Angelman syndromes;
deletion of GABRB3 is found in both disorders and it is, therefore, suggested that loss of the β3 subunit plays
a role in the pathogenesis of these syndromes

GABA-A gamma 1 (γ1)

GABRG1

both the γ1 and γ2 subunits are important in the effects of the benzodiazepines on GABA-A
receptor function;

GABA-A gamma 2 (γ2)

GABRG2

both the γ1 and γ2 subunits are important in the effects of the benzodiazepines on GABA-A
receptor function; polymorphisms in the GABRG2 gene are associated with susceptibility to epilepsy
and febrile seizures; presence of the γ2 subunit results in a low sensitivity of GABA-A
receptors to allosteric regulation by zinc ion

GABA-A gamma 3 (γ3)

GABRG3

the γ3 subunit is critical to the allosteric regulation of GABA-A receptors by zinc ions whereas
presence of the γ2 subunit results in a low sensitivity to zinc ion regulation

GABA-A delta (δ)

GABRD

polymorphisms in the GABRD gene are associated with susceptibility to epilepsy and febrile seizures;
three variants of the GABRD protein are produced in the brain identified as GABRD-1A, -1B, and -1C;
the δ subunit is involved in the tonic (continuous) currents elicited by GABA-A receptors which
modifies the spatial and temporal integration of excitatory neurotransmission

GABA-A epsilon (ε)

GABRE

alternative splicing of the GABRE mRNA occurs at several positions depending upon the tissue
of expression

GABA-A pi (π)

GABRP

expressed at highest levels in the uterus; presence of the π subunit in pentameric
GABA-A receptors modifies the receptor sensitivity to steroidogenic compounds

GABA-A theta (θ)

GABRQ

GABA-A rho 1 (ρ1)

GABRR1

protein contains a chloride-sensitive anion channel

GABA-A rho 2 (ρ2)

GABRR2

GABA-A rho 3 (ρ3)

GABRR3

GABA also acts on GABA-B receptors that are members of the GPCR family of receptors. There are
two GABA-B receptors subunits identified as GABA-B1 (GABAB1) and GABA-B2 (GABAB2).
These two subunits heterodimerize to form the functional receptor that can be found on both
pre- and post-synaptic membranes. Neither receptor subunit is functional as a GABA
receptor independently. The GABA-B receptors are coupled to G-proteins of the Gi type.
The G-protein is linked to potassium channels (GIRK or Kir3) and activation of the G-protein results in
increased conductance of the associated channel. GABA-B receptor activation on post-synaptic membranes generally
leads to activation of the inwardly rectifying potassium channels which underlies the late
phase of inhibitory postsynaptic potentials (IPSPs). Activation of pre-synaptic GABA-B receptors
decreases neurotransmitter release by inhibiting voltage-activated Ca2+ channels of the N or P/Q types.
Activation of GABA-B receptors also modulates the production of cAMP. This function leads to a wide range
of actions on ion channels as well as other proteins that are targets of PKA. The cAMP modulation
by GABA-B receptors effects modulation of both neuronal and synaptic functions.

Pharmacology of GABA Receptors

The anxiolytic/sedative effects of the barbiturates and benzodiazepines are exerted via their
binding to subunits of the GABA-A receptors. Benzodiazepines bind to a site on the GABA-A receptor
created by the association of the gamma (γ) subunit and one of the the alpha (α) subunits. There
are two distinct subtypes of benzodiazepine receptors termed BZ1 (BZ1) and BZ2
(BZ2). The BZ1 receptor is formed by the interaction of γ and α1 subunits,
whereas the BZ2 receptors is formed by the interaction of the γ and α2, α3 or
α5 subunits. The receptor for the barbiturates is the beta (β) subunit of the GABA-A
receptor. When benzodiazepines bind to
the GABA-A receptor they potentiate the actions of GABA and require the presence of GABA in order for
activation of the ion channel. Barbiturates can induce GABA-A channel opening in the absence of GABA
when administered at high dose and as a result they can be lethal due to the level of CNS suppression.
The potential for lethal toxicity of a benzodiazepine requires an extremely large dose. This difference in toxicity between
barbiturates and benzodiazepines is the major reason barbiturates are not often used clinically any longer.
The significance of the BZ1 receptor isoform is that it is solely involved in mediating the induction
of sleep. This fact has led to the development of several classes of drug that specifically
target this GABA-A receptor isoform, and more precisely, the site on the GABA-A complex that forms the
BZ1 binding site. The non-benzodiazepine drug, zolpidem (Ambien®), exerts its hypnotic sleep inducing
effects due to near selective binding to the BZ1 site. Another non-benzodiazepine drug used for its
hypnotic sleep inducing effect is eszopiclone (Lunesta®). Although the precise mechanism of action
of eszopiclone is not fully understood, it is believed to function similarly to zolpidem in binding
to the BZ1 receptor site on GABA-A receptor isoforms.

Acetylcholine

Acetylcholine (ACh) is a simple molecule synthesized
from choline and acetyl-CoA
through the action of choline acetyltransferase.
Neurons that synthesize and release ACh are termed cholinergic neurons. When an action
potential reaches the terminus of a presynaptic neuron a voltage-gated calcium channel is opened. The influx of calcium ions,
Ca2+, stimulates the exocytosis of presynaptic vesicles containing ACh,
which is thereby released into the synaptic cleft. Once released, ACh must be removed rapidly in order to allow
repolarization to take place; this step, hydrolysis, is
carried out by the enzyme, acetylcholinesterase (AChE). AChE is a highly active
enzyme capable of hydrolyzing on the order of 25,000 molecules of ACh per
second. The released choline is then taken back up by the presynaptic neuron
where it can once again serve as a substrate for ACh synthesis via choline
acetyltransferase.

Two different mammalian AChE isoforms are produced from the
single ACHE gene (chromosome 7q22.1) in humans via alternative splicing and post-translational
modification. These two AChE isoforms are termed T (tail) and H (hydrophobic).
The T form (AChET, also known as the hydrophilic form) is the predominant enzyme
in the brain and at the neuromuscular junction. The H form (AChEH) is the
principal enzyme form found in erythroid cells. The AChEH isoform is anchored to
red blood cell membranes via a GPI-linkage
and this form constitutes the Yt
blood group antigen.

Synthesis of Acetylcholine

Two main classes of ACh receptors have been identified on the basis of their
responsiveness to the toadstool alkaloid muscarine and to nicotine, respectively.
The muscarinic receptors (mAChRs) and the nicotinic
receptors (nAChRs). The muscarinic receptors are G-protein coupled receptors
(GPCR) and are also referred to as metabotropic receptors. The nicotinic receptors are
ligand-gated ion channels which are also referred to as ionotropic receptors.
Both receptor classes are abundant in the human brain.

Muscarininc Acetylcholine Receptors

The are five subtypes of muscarinic receptors, identified as M1–M5, that are
classified based upon pharmacological activity. The M1, M3, and M5 muscarinic
receptors are coupled to the Gq type G-proteins that activate PLCβ. The M2 and
M4 receptors are coupled to Gi type G-proteins that inhibit adenylate cyclase.
Muscarinic receptor desensitization occurs in response to phosphorylation of the receptors by kinases that
are members of the G-protein coupled receptor kinase (GRK) family. For example the M2 receptor is phosphorylated
by GRK2 (originally called β-adrenergic receptor kinase-1, βARK1). More information on
the GRK family can be found in the Signal Transduction page.

coupled to a Gq/11-type G-protein; M1 receptor agonists cause epileptic seizures; loss
of M1 receptor function results in increased dopamine release from striatum, suggests that
pharmacological blockade of this receptor may be useful in Parkinson disease

M2 (M2)

CHRM2

predominant receptor in heart tissue; lungs; smooth muscle

coupled to a Gi-type G-protein; activates K+-channel
as well as decreasing cAMP; M2 receptor agonists induce analgesia with much less
risk of addiction relative to opioid analgesics; M2 receptor responsible for
cholinergic deceleration of cardiac rate

M3 (M3)

CHRM3

broadly expressed throughout the brain at low levels; expressed in periphery
in secretory glandular tissues and smooth muscle cells; expressed on parietal cells of
stomach

coupled to a Gq/11-type G-protein; important for contraction of smooth
muscle in the urinary bladder, ileum, stomach fundus, trachea and gallbladder;
ACh binding to parietal cell M3 receptors induces mobilization of proton (H+)
pump migration to lumenal membrane for gastric acid production in stomach

M4 (M4)

CHRM4

abundantly expressed in striatum; lungs

coupled to a Gi-type G-protein; activates K+-channel
as well as decreasing cAMP; locomotor activity increased by pharmacologic
blockade of the M4 receptor

Nicotinic Acetylcholine Receptors

Nicotinic receptors are divided into those found at neuromuscular junctions and
those found at neuronal synapses. The nicotinic receptors are
composed of five types of subunits which are found in different combinations in
different types of nicotinic receptors. There are 16 known nAChR subunit genes in the human genome
that encode the alpha (α1–α7, α9, and α10), beta
(β1–β4), delta (δ), epsilon (ε), and gamma (γ) subunits. The
alpha subunit genes are designated CHRNA1–CHRNA7, CHRNA9, and CHRNA10. The beta genes are
CHRNB1–4, while the delta, gamma, and epsilon genes are CHRND, CHRNG, and CHRNE,
respectively. Regardless of subunit composition or cellular location, all of the nAChRs are pentameric
receptors. All of the nAChRs are divided into two broad categories: neuromuscular-type and neuronal-type.
Regardless of type, nAChRs that contain the α4 and β2 subunits are the highest affinity receptors.

There are two major types of neuromuscular nicotinic receptors; one
is composed of α1, β1, δ, and ε subunits (referred to as the embryonic form)
while the other is composed of α1,
β1, δ, and γ (referred to as the adult form).
There are five types of neuronal receptors, with one of the latter type also found in epithelial tissues.
The neuronal nAChRs are only composed of various α and β subunits making up the
pentameric receptor. For example, the ganglion nAChR is comprised of an (α3)2(β4)3
pentameric arrangement.

The activation of nicotinic acetylcholine receptors by the binding of ACh
leads to an influx of Na+ into the cell and
an efflux of K+, resulting in a depolarization of the postsynaptic
neuron and the initiation of a new action potential. Desensitization of the nAChRs occurs as a result
of phosphorylation by either PKA or PKC.

Cholinergic Agonists and Antagonists

Numerous compounds have been identified that act as
either agonists or antagonists of cholinergic neurons. The principal action of
cholinergic agonists is the excitation or inhibition of autonomic effector cells that are innervated by postganglionic
parasympathetic neurons and as such are referred to as
parasympathomimetic agents. The cholinergic agonists include choline esters (such as ACh
itself) as well as protein- or alkaloid-based compounds. Several naturally
occurring compounds have been shown to affect cholinergic neurons,
either positively or negatively.

The responses of cholinergic neurons can also be
enhanced by administration of cholinesterase (ChE)
inhibitors. ChE inhibitors have been used as
components of nerve gases but also have significant medical application in the
treatment of disorders such as glaucoma and myasthenia gravis as well as in
terminating the effects of neuromuscular blocking agents such as atropine.

Cholinergic Pharmacology

Pharmacological intervention in the functions of acetylcholine is effected by either of two routes. In the direct-acting class there are the acetylcholine mimicking drugs (cholinomimetics) and in the indirect-acting class are the acetylcholinesterase inhibitors. There are numerous cholinomimetic drugs which includes methacholine, carbachol, and bethanechol as prominent examples. Methacholine exerts its effects through the muscarinic acetylcholine receptors and is used primarily in the bronchial challenge test used to diagnose hyperactivity in the bronchial tree as it typical in asthma. Carbachol functions primarily by activating nicotinic acetylcholine receptors and can exert systemic effects in the gastrointestinal system and in the bladder. However, it is used primarily as a locally administered drug in the treatment of glaucoma. Bethanechol is a muscarinic acetylcholine receptor agonis used primarily in the treatment of urinary retention following anesthesia and in diabetic neuropathy.

Acetylcholinesterase (AChE) inhibitors are used to increase the effective level and action of acetylcholine acting at both muscarinic and nicotinic acetylcholine receptors. Aceythcholinesterase inhibitors that are used pharmacologically are principally of the reversible (competitive and noncompetitive) type. Irreversible AChE inhibitors exert toxic effects such as the effects of the toxic organophosphate pesticides and nerve agents. The reversible AChE inhibitors, such as donepezil, rivastigmine and galantamine, are commonly used in the treatment neurodegenerative disorders such as Alzheimer disease (AD) and Parkinson disease (PD). These reversible inhibitors are also used to treat the neuromuscular disorder, myasthenia gravis, that results from autoimmune destruction of nicotinic acetylcholine receptors (nAChR). Reversible AChE inhibitors are also used as antidotes in the treatment of organophosphate pesticide intoxication. Although these AChE inhibitors are used to reduce the symptoms associated with AD they do not exert their effects in the long term (being effective for only 12-24 months) and they have no effects on the rate of cognitive decline in AD. The carbamates (derived from carbamic acid: NH2COOH) represent a large class of compounds, many of which are reversible AChE inhibitors (e.g. rivastigmine). Although reversible, the carbamates can exert acute toxic effects that are similar to those of the irreversible organophosphates. Indeed, several carbamate compounds are used as pesticides and parasiticides in the veterinary field. Clinically the carbamates are used in the treatment of myasthenia gravis, glaucoma, and neurodegenerative disorders such as AD and PD, similarly to donepezil and galantamine. Although the irreversible AChE inhibitors are quite toxic and have been used as deadly nerve agents (e.g. VX) and as insecticides, they do have pharmacologic utility. The drug echothiophate (phospholine) is administered locally in the treatment of glaucoma and metrifonate is used in the treatment of AD and PD.

As described above, the neurotransmitters and receptors of the parasympathetic nervous system are those of the cholinergic family. The principal neurotransmitter is acetylcholine (ACh) and the receptors are the muscarinic acetylcholine receptors M2 and M3. For example, the primary vascular response to ACh binding to M3 receptors on endothelial cells is the activation of nitric oxide synthase (NOS) and the production of nitric oxide (NO). However, it is important to note that the endothelial M3 receptor is not innervated by cholinergic nerve fibers, but responds to the binding of circulating ACh. Production of NO results in relaxation of the smooth muscle cells leading to vasodilation. Nicotinic ACh receptors are located postsynaptically in all autonomic ganglia and at the neuromuscular junction (NMJ). At the NMJ, nicotinic receptors function as the excitatory receptor for the postsynaptic cell.

As pointed out in the introduction to this page, neurotransmission within the sympathetic and parasympathetic ganglia
involves the release of ACh from preganglionic efferent nerves. Once released, the ACh then binds to nicotinic receptors
in the membrane of the cell bodies of the postganglionic efferent nerves. Ganglionic blockers (primarily nicotinic ACh receptor antagonists) are drugs that function
by inhibiting autonomic activity via interference with the transmission of nerve impulses within autonomic ganglia.
Therefore, ganglionic blockers reduce sympathetic outflow. With respect to cardiac tissue, ganglionic blockade results in decreased
cardiac output due to both decreased chronotropic (heart rate) and inotropic (contraction strength) activity.
Ganglionic blockers also lead to reduced sympathetic output to the vasculature resulting in decreased sympathetic
vascular tone. This latter effect causes vasodilation and reduced systemic vascular resistance resulting in decreased
arterial pressure. It is important to note that parasympathetic nerve transmission (outflow) is also reduced by ganglionic
blocking drugs. For this reason, as well as the development of more highly
selective drugs for the treatment of hypertension, ganglionic blockers (e.g.
mecamylamine and hexamethonium) are not commonly used any longer in the
treatment of hypertension.

Catecholamines: Dopamine, Epinephrine, Norepinephrine

The principal catecholamines
are norepinephrine, epinephrine
and dopamine. These compounds are formed from the amino acid tyrosine. Tyrosine is produced, primarily, in the liver from phenylalanine through the
action of phenylalanine hydroxylase. The tyrosine is
then transported to catecholamine-secreting neurons where a series of reactions
convert it to dopamine, to norepinephrine and finally
to epinephrine (see also Specialized
Products of Amino Acids). Within the substantia nigra locus of the brain, and some
other regions of the brain, synthesis proceeds only to dopamine. Within the locus coeruleus region of the brain the end product of the pathway is norepinephrine. The presence of high concentrations of tyrosine in the locus coeruleus and the substantia nigra leads to increased melanin synthesis which confers on these brain regions a dark bluish coloration observable in brain sections. Indeed, these brain regions are so-called due to the dark bluish-black pigmentation. The Latin term, substantia nigra, means "black substance". The Latin word coeruleus means "dark blue, blue, or blue-green". Within adrenal medullary chromaffin cells, tyrosine is converted to norepinephrine and epinephrine.

Synthesis of the catecholamines from tyrosine. Tyrosine is converted to each of the three catecholamines through a series of four reactions. The tissue from which the neurotransmitter/hormone is derived expresses a specific set, or all, of these enzymes such that only dopamine (substantia nigra) is the result, or only norepinephrine (locus coeruleus), or both norepinephrine and epinephrine (adrenal medulla). DOPA decarboxylase (also known as aromatic L-amino acid decarboxylase) is encoded by the DDC gene. Dopamine β-hydroxylase is a critical vitamin C (ascorbate) and copper (Cu2+)-dependent enzyme.

Once synthesized, dopamine, norepinephrine and epinephrine are packaged in granulated vesicles for secretion in response to the appropriate nerve impulse. Within these vesicles, norepinephrine and epinephrine are bound to ATP and a protein called chromogranin A. Norepinephrine is the principal neurotransmitter of sympathetic postganglionic nerves. Both norepinephrine and epinephrine are stored in synaptic knobs of neurons that secrete it, however, epinephrine is not a mediator at postganglionic sympathetic nerve impulses. The major location, within the brain, for norepinephrine synthesis is the locus coeruleus of the brainstem. The major brain region for the synthesis of dopamine is the substantia nigra which is located below the posterior hypothalamus and next to the ventral tegmetal area. Outside the brain, the major site of norepinephrine and epinephrine synthesis is in adrenal medullary chromaffin cells. Outside the brain, dopamine is synthesized in several tissues including the gastrointestinal system where its actions reduce gastrointestinal motility, the pancreas where its actions inhibit insulin synthesis, and in the kidneys where its actions increase sodium excretion and urinary output.

Catecholamines exhibit peripheral nervous system excitatory and inhibitory effects as well as actions in the CNS such as respiratory stimulation and an increase in psychomotor activity. The excitatory effects are exerted upon smooth muscle cells of the vessels that supply blood to the skin and mucous membranes. Cardiac function is also subject to excitatory effects, which lead to an increase in heart rate and in the force of contraction. Inhibitory effects, by contrast, are exerted upon smooth muscle cells in the wall of the gut, the bronchial tree of the lungs, and the vessels that supply blood to skeletal muscle. In addition to their effects as neurotransmitters, norepinephrine and epinephrine can influence the rate of
metabolism. This influence works both by modulating endocrine function such as
insulin secretion and by increasing the rate of glycogenolysis
and fatty acid mobilization.

The primary effects of the catecholamines are exerted as neurotransmitters upon their stimulated release from presynaptic nerve terminals in the appropriate target organ. However, release of the catecholamines from adrenal medullary cells to the systemic circulation allow them to function as hormones as well. Regardless of their site of release, the catecholamines exert their effects by binding to receptors of the G-protein coupled receptor (GPCR) family. The catecholamines are also known as adrenergic neurotransmitters and the neurons that secrete them are referred to as adrenergic neurons. Norepinephrine-secreting neurons are specifically termed noradrenergic neurons. Some of the norepinephrine released from presynaptic noradrenergic neurons is recycled in the presynaptic neuron by a reuptake mechanism similar to that responsible for regulating the CNS actions of serotonin.

The actions of norepinephrine and epinephrine are exerted upon binding to and activating the adrenergic receptors of which there are nine distinct forms. As indicated, the adrenergic receptors are all members of the GPCR family. There are two distinct types of
adrenergic receptor identified as the α (alpha) and β (beta) receptors. In addition, there are two functionally distinct classes of α adrenergic receptor identified as the α1 and α2 forms. Within each α-adrenergic receptor type there
are several variants encoded by distinct genes as well as additional isoforms that result from alternative mRNA splicing. The α1 receptors consist of the α1A, α1B, and α1D receptors. The α1 receptors are coupled to Gq-type G-proteins that activate
PLCβ resulting in increases in IP3 and DAG release from membrane PIP2. The α2
receptors consist the α2A, α2B, and α2C receptors. The α2 receptors are coupled to Gi-type G-proteins that inhibit the activation of adenylate cyclase and therefore, receptor activation results in reduced levels of cAMP
and consequently reduced levels of active PKA. The β adrenergic receptors are composed of three types: β1, β2,
and β3 each of which couple to Gs-type G-proteins resulting in activation of adenylate cyclase and increases in cAMP with concomitant activation of PKA. However, the β2 receptor can switch from Gs to Gi/o signaling following phosphorylation of the receptor by PKA.

Dopamine binds to dopamineric receptors identified as D-type receptors and
there are five receptors identified as D1, D2, D3, D4, and D5.
All five dopamine receptors belong the the G-protein coupled receptor (GPCR) family. The D1 and D5
dopamine receptors are coupled to the activation of Gs-type G-proteins and, therefore, receptor activation results in
activation of adenylate cyclase. The D2, D3, and D4 dopamine receptors are coupled to Gi-type G-proteins and, therefore, receptor activation results in the inhibition of adenylate cyclase. The D1 and D5 receptors constitute members of the D1-like receptor family. The D2, D3, and D4 receptors constitute members of the D2-like receptor family.

Adrenegic and Dopaminergic Receptors

Receptor Type

Gene Symbol(s)

Expression Profile

Functions / Comments

α1

ADRA1A
ADRA1B
ADRA1D

predominates in vascular smooth muscles in the vessels of the skin, gastrointestinal system, kidneys, and central nervous system (CNS); also expressed in
adipose tissue

abundant in adipocytes of BAT and omental fat, gallbladder and bladder, is not
expressed in the heart, skeletal muscle, liver, kidneys, lung, or thyroid gland

coupled to Gs-type G-protein, regulation of lipolysis, principal norepinephrine receptor in BAT,
increase lipolysis in BAT and plays major role in adaptive themogenesis

D1

DRD1

expressed at a high level of density in the nigrostriatal, mesolimbic, and mesocortical areas,
such as the caudate-putamen (striatum), nucleus accumbens, substantia nigra, olfactory bulb,
amygdala, and frontal cortex, lower levels in the hippocampus, cerebellum, thalamic
and hypothalamic areas; kidney

selectively associated with the limbic system of the CNS such as the shell of the nucleus accumbens
and the olfactory tubercle; not expressed outside CNS

coupled to Gi-type G-protein, together with D2 and D4 receptors forms the
D2-like family, limbic system receives dopamine inputs from the
ventral tegmental area which is associated with cognitive, emotional, and endocrine functions; regulation
of locomotor effects; modulation of cognitive functions

Adrenergic Pharmacology

With respect to the sympathetic nervous system (see above), the principal neurotransmitters
are norepinephrine and epinephrine and the receptors
are α1, β1, and β2. Alpha-adrenergic receptors of the
sympathetic nervous system play important roles in cardiac and vascular function. The presence of
the α1 receptor in arteries causes them to constrict upon binding epinephrine
or norepinephrine. This effect results in increased blood pressure and increased blood
flow returning to the heart. Significantly, however, is the fact that the blood vessels in skeletal
muscles lack α1 receptors so that they can remain open to utilize the increased blood
pumped by the heart, particularly in response to stress.

Adrenegic Receptor Drugs

Drug Class

Major Target(s)

Effects / Comments

non-selective α blockers

both classes of α receptor; primary responses at a1 receptors

reduce vasoconstriction caused by both norepinephrine and epinephrine acting at α1
receptors; inhibition of α2 receptors exerted within sympathetic nervous system;
phenoxybenzamine is most well known in this class, was once also used to treat
benign prostatic hyperplasia (BPH); yohimbine is used as an herbal treatment for erectile dysfunction but excess usage can lead to
tachycardia, dizziness, and anxiety

selective α blockers

α1 receptors

all drugs in this class
end with the suffix 'osin' (e.g. prazosin); drugs in this class can induce orthostatic hypotension;

α1 agonists

α1 receptors

function systemically to increase blood pressure; methoxamine and phenylephrine increase peripheral vascular resistance resulting in increased blood pressure; these drugs are used in the treatment of hypotension and shock

α2 agonists

α2 receptors

function centrally (i.e. within the CNS) to lower blood pressure; methyldopa is longest
used drug in this class, although not used often any longer it is still the drug of choice (DOC) for
the treatment of hypertension in pregnancy; cause vasodilation and reduction in blood pressure; also used to treat BPH; although classified as a non-selective agonist, clonidine is widely used drug for the treatment of hypertension and hot flashes in menopausal females; clonidine also approved for the treatment of attention deficit hyperactivity disorder (ADHD)

β1 blocker drugs (e.g. metoprolol) all have a name with the suffix
‘olol'; β1-specific blockers function to decrease heart rate (chronotropic effects) and
contractility (inotropic effects) leading to decreased blood pressure; β blockers are also sometimes prescribed for the treatment of glaucoma, migraine headaches, and anxiety

β2 agonists

β2 receptors

β2 agonists are principally used to induce bronchodilation in asthmatics and others with
pulmonary dysfunction; these drugs are divided into the short-acting and long-acting subtypes; salbutamol
is the most common of the prescribed short-acting drugs; all of the
long-acting drugs have a name with the suffix ‘terol' (e.g. formoterol)

When considering the effects of various adrenergic receptor agonist and
antagonist effects within the vasculature it is important to understand that the contractile
characteristics and the mechanisms that cause contraction of
cardiac myocytes and vascular smooth muscle (VSM) are very distinct. The
contractile properties of cardiac myocytes are fast and of extremely short
duration. In contrast, VSM undergoes slow, sustained, tonic contractions. While
both cardiac muscle and VSM contain actin and myosin, VSM do not express the regulatory troponin
complex as
do striated muscle cells such as cardiac myocytes. An additional difference between VSM and cardiac myocytes relates
to the structural arrangement of actin and myosin. In heart muscle cells these
proteins are organized into distinct bands, whereas, in VSM they are not.
Although organized differently, the contractile proteins of VSM are indeed highly organized
in order to allow for maintaining tonic contractions and reducing vascular
diameter.

Contraction of VSM is initiated by by several distinct phenomena including mechanical, electrical, and chemical stimuli.
Mechanical contraction refers to the passive stretching of VSM from the cell itself and is therefore termed a myogenic response. Electrical
stimulation of VSM contraction involves depolarization of the membrane, most
often as a result of the opening of voltage gated calcium channels (L-type calcium channels)
leading to increased intracellular calcium concentrations. When discussing
chemical stimuli, that initiate contraction in VSM, these signals are hormones and
neurotransmitters such as epinephrine and norepinephrine, angiotensin II, vasopressin
(anti-diuretic hormone, ADH), endothelin-1, and thromboxane A2 (TXA2).
Each of these molecules binds to specific receptors on the VSM cell or to
receptors on the endothelial cells adjacent to the VSM. The consequences of
receptor activation are VSM contraction. Although each of these
receptor-mediated VSM contraction processes are different, they converge at the
point of increased intracellular calcium concentration.

Increases in free intracellular calcium result from either increased calcium
influx into the VSM or via the release of sarcoplasmic reticulum (SR) stored
calcium. Within the VSM cell, free calcium binds to the regulatory protein, calmodulin. Calcium-calmodulin activates myosin light chain kinase (MLCK)
which then phosphorylates myosin light chains. Phosphorylation of myosin light
chains induces the formation of cross-bridges between the myosin heads and the actin filaments
leading to smooth muscle contraction.
Conversely, relaxation of VSM cells occurs in response to reduced levels of
myosin light chain phosphorylation. Adrenergic receptor stimulation by
epinephrine or norepinephrine involves G-protein-coupled signal transduction pathways
that impinge upon levels of the PKA activating molecule, cAMP. Since α1
receptors are coupled to the activation of Gq proteins there is a resultant
increase in release of intracellular calcium via the action of the second messenger IP3
binding to SR membrane receptors. The consequences of the released calcium are, therefore, VSM contraction.
Norepinephrine is the major activator of α1
receptors. Norepinephrine also activates α2 receptors which are Gi coupled receptors. The
resultant inhibition of cAMP production due to the inhibition of adenylate
cyclase leads to increased MLCK activity. The effects, therefore, of
norepinephrine at α1 and α2 receptors are the same but elicited via different
signaling pathways. On the other hand, epinephrine activates β2 receptors which are coupled to
Gs proteins which activate adenylate cyclase resulting in increased cAMP concentrations.
In most cells an increase in cAMP leads to an increase in the activity of the kinase, PKA. Although it would seem
counterintuitive for this pathway to be activated under conditions where VSM relaxation was needed, the increased
cAMP levels induced by VSM β2 receptor activation result in inhibition of MLCK, thereby reducing
myosin light chain phosphorylation. In addition, activated PKA phosphorylates a membrane potassium channel (KATP)
in VSM resulting in hyperpolarization of the cell preventing the Ca2+ influx that is required for
contraction. The net effect of both of these β2 receptor-medicated events is VSM relaxation.

Activation of the β1
receptor in the heart results in an increase in both the inotropic (heart rate) and the chronotropic (strength of contraction)
activity of the heart muscle. Pharmacologic antagonism of the
β1 receptor in the heart, such as with metoprolol (or any other of this drug class;
identifiable by the ‘olol' ending), results in decreasing heart rate and contractility.
The overall effect is a decrease in blood pressure. This is the basis for the use of beta blocker
drugs in the treatment of hypertension and to decrease the chance of a dysrhythmia after a heart attack.
The β2 receptors are prevalent in the smooth muscle cells of the bronchioles
of the lungs and arteries of skeletal muscle. Activation of
the β2 receptors in bronchioles causes them to dilate which allows more oxygenated
air to enter the lungs. Simultaneously, activation of β2 receptors in the arteries of skeletal muscle
causes them to dilate to allow increased blood flow
into this tissue. Both of these receptor-mediated activities allow for an enhanced response to stress such as is typical
of the fight-or-flight response. It is important to note that norepinephrine
also binds weakly to β2 receptors which results in vasodilation as for the case of epinephrine. This phenomenon is
most noticeable pharmacologically when α1 blockers such as
prazosin (drugs in this class all end in 'osin') are utilized. Under normal
physiological conditions this vasodilator effect of norepinephrine is overwhelmed by α1
receptor-mediated vasoconstriction. Equally important is the fact that, although
epinephrine binds with highest affinity to VSM β2 receptors to induce vasodilation,
at high concentrations it will bind to α1 and α2 receptors which can override β2
receptor effects leading to vasoconstriction.

Catecholamine Catabolism

Epinephrine and norepinephrine
are catabolized to inactive compounds through the
sequential actions of catecholamine-O-methyltransferase
(COMT) and monoamine oxidase (MAO). Compounds that
inhibit the action of MAO have been shown to have beneficial effects in the
treatment of clinical depression, even when tricyclic
antidepressants are ineffective. The utility of MAO inhibitors was discovered
serendipitously when patients treated for tuberculosis with isoniazid
showed signs of an improvement in mood; isoniazid was
subsequently found to work by inhibiting MAO.

Dopamine: Reward Reinforcement and Feeding Behaviors

Overall control of feeding behavior is a complex process involving several
well deﬁned neural circuits.
These circuits consist of interactions between the brainstem and the hypothalamus as well as
interactions between the gut and the hypothalamus. For detailed information on the latter go to the
Gut-Brain Interrelationships page.
The control of feeding behavior also involves overlapping processes such as motivational drive, satiety
and the anticipation of food. A major neurotransmitter involved in the coordination and reinforcement
of these reward processes is dopamine. Indeed, every known type of reward, including food, results in increased levels of
dopamine in the brain. Although the cell bodies of dopaminergic neurons are confined to only a few
areas of the brain, these neurons send projections to numerous areas including those involved in the
regulation of feeding behaviors such as the hypothalamus.

Dopamine mediates the motivational and rewarding aspects of food seeking behavior via specific
dopaminergic projections from the ventral tegmental area (VTA) to the nucleus acumbens (NAc).
The VTA is the origin of the dopaminergic cell bodies and the NAc is a
brain region in the basal forebrain that sends projections to the basal ganglia situated at the
base of the forebrain. The NAc is involved in reinforced learning, reward, pleasure, addiction,
fear, aggression, and impulsivity. The reward pathways involving dopamine are also referred to as
the mesolimbic or mesocorticolimbic system which also sends projections to the medial prefrontal cortex,
hippocampus and amygdala. The mesolimbic dopamineric circuits are involved in the motivation to
earn food rewards but not for the triggering of actual food consumption. Dopamine also mediates
food consumption by sending projections from the substantia nigra to the dorsolateral striatum.
Although mesolimbic dopamine circuits have clearly been associated with reward processes, the specifics
of its involvement in the process are quite complex. It is important to be able to distinguish between
the diverse aspects of motivational function that are differentially affected by dopamine activity.
Pharmacological manipulation of dopamine demonstrates that mesolimbic dopamine is indeed critical for many
aspects of motivational function, but also that it is not critically involved in all aspects of motivational function.
In addition, some of the effects of mesolimbic dopamine are linked to aversive motivation and learning.
However, the studies on the fundamental characteristics of reinforcing stimuli have concluded that mesolimbic dopaminergic
signals, acting as positive reinforcers, tend to be preferred and thus, elicit approach, goal-directed, and high demand
behaviors characteristic of positive reinforcement.

In addition to direct dopaminergic neuronal actions, the activity of the mesolimbic system is
modulated by peripheral hormones that are known to regulate feeding behaviors via hypothalamic circuits
such as leptin and ghrelin. Leptin
is an anorexigenic hormone (decreases desire for food) produced
by adipose tissue whereas, ghrelin
is an orexigenic hormone (increase desire for food) produced by the stomach.
Leptin action in the VTA results in reduced firing of dopaminergic neurons
and decreases food intake. Conversely, animal studies demonstrate that loss of leptin receptors in the
VTA leads to increased food intake. Ghrelin receptors are present in the VTA and NAc and activation of these receptors
leads to increased food intake. These observation reinforce the role of dopamine
in feeding behavior and demonstrate the interconnections between peripheral and
central neurotransmitter actions in overall regulation of feeding.

The greatest concentration of 5HT (90%) is found in the enterochromaffin
cells of the gastrointestinal tract. Most of the remainder of the body's 5HT is
found in platelets and the CNS. Platelets themselves are incapable of
synthesizing serotonin but acquire it from plasma via the action of the
serotonin transporter (SLC6A4).
The effects of 5HT are felt most prominently in
the cardiovascular system, with additional effects in the respiratory system
and the intestines. Vasoconstriction is a classic response to the
administration of 5HT.

Neurons that secrete 5HT are termed serotonergic. Following the
release of 5HT, a portion is taken back up by the presynaptic
serotonergic neuron in a manner similar to that of
the reuptake of norepinephrine.

The function of serotonin is exerted upon its
interaction with specific receptors. At least 15 serotonin receptors have been
cloned and are identified as 5HT1, 5HT2, 5HT3,
5HT4, 5HT5, 5HT6, and 5HT7. All of
the serotonin receptor genes in this superfamily are abbreviated as HTR followed
by a numeral and letter designation. Within
the 5HT1 group there are subtypes HTR1A, HTR1B, HTR1D, HTR1E (a
putative 5HT receptor), and HTR1F. There are three 5HT2
subtypes, HTR2A, HTR2B, and HTR2C (was originally identified as the 5HT1C
receptor). There are five 5HT3 subtypes, HTR3A, HTR3B, HTR3C,
HTR3D, and HTR3E. The receptors HTR3C, HTR3D, and HTR3E are referred to as serotonin-like receptors.
There are two 5HT5 subtypes, HTR5A and HTR5B in the human
genome but the HTR5B gene is a pseudogene. Most of these receptors are coupled to G-proteins that
affect the activities of either adenylate cyclase or phospholipase Cβ (PLCβ). The 5HT3 class of
receptors are ion channels (ionotropic receptors).

Some serotonin receptors are presynaptic
and others postsynaptic. The 5HT2A receptors mediate platelet
aggregation and smooth muscle contraction. The 5HT2C receptors are
suspected in control of food intake as mice lacking this gene become obese from
increased food intake and are also subject to fatal seizures. The 5HT3
receptors are present in the gastrointestinal tract and are related to
emesis (vomiting). Also present in the gastrointestinal tract are 5HT4 receptors
where they function in secretion and peristalsis. The 5HT6 and 5HT7
receptors are distributed throughout the limbic system of the brain and the 5HT6
receptors have high affinity for antidepressant drugs.

anxiety, locomotion, gastrointestinal motility, blood pressure, appetite
(by modulating melanocortin pathways), mood, sexual behavior, erectile function,
thermoregulation, sleep, addictive behaviors; agonists may hold promise in treating obesity;
pre-mRNA is edited at five sites
[A, B, C' (E), C, and D]; in victims of suicide, the level of C' editing is much higher and the level of D
editing is significantly decreased when compared in unaffected individuals, in mice
treated with the antidepressant, fluoxetine, the
pattern of C, C', and D editing is the exact opposite to that observed in victims of suicide

5HT3A

HTR3A

predominantly expressed in small intestine, colon, and CNS; low levels detected in spleen, thymus, and prostate

Histamine

Histamine Synthesis and Catabolism

Histamine is referred to as a biogenic amine which is a potent neurotransmitter that binds to specific histamine receptors (see below). Histamine is synthesized by the enzymatic decarboxylation of the amino acid histidine by the enzyme L-histidine decarboxylase (HDC). Within the gastrointestinal tract bacteria also produce histamine via a similar decarboxylation reaction. The principal cells that synthesize and release histamine are mast cells and basophils of the immune system, enterochromaffin-like cells of the gastrointestinal system, and neurons. The synthesis and storage of histamine by mast cells and basophils represents the greatest store (>90%) of the neurotransmitter. Within the brain the neurons that synthesize histamine are within the tuberomammillary nucleus of the hypothalamus.

Synthesis of Histamine

The histidine decarboxylase gene (symbol: HDC) is located on chromosome 15q21.2 and is composed of 14 exons that generate two alternatively spliced mRNAs encoding two distinct isoforms of the enzyme. The isoform 1 protein is composed of 662 amino acids and the isoform 2 protein is composed of 629 amino acids.

Following its release, histamine is metabolized by two alternative pathways, one of which is an oxidation and the other a methylation reaction. The oxidation of histamine is catalyzed by a diamine oxidase encoded by the AOC1 (amine oxidase, copper containing 1) gene. The product of diamine oxidase reaction is imidazole acetaldehyde which is itself further metabolized to imidazole acetate via the action of an aldehyde dehydrogenase. In addition to the oxidation of histamine the diamine oxidase enzyme is involved in the metabolism of
putrescine. The methylation of histamine is catalyzed by the enzyme histamine N-methyltransferase encoded by the HNMT gene. The methyl group utilized by the HNMT encoded enzyme is derived from S-adenosylmethionine. The product of the methyl transfer reaction is N-methylhistamine which is then further metabolized to N-methylimidazole acetate via the action of monoamine oxidase, MAO. MAO is also critically involved in the metabolism of the catecholamines.

Histamine Receptors

Humans express four distinct histamine receptors identified as H1R, H2R, H3R, and H4R. All four histamine receptors are members of the
G-protein coupled receptor superfamily. The H1R protein is encoded by the HRH1 gene, the H2R protein is encoded by the HRH2 gene, the H3R protein is encoded by the HRH3 gene, and the H4R protein is encoded by the HRH4. The HRH1 gene is located on chromosome 3p25 and is composed of 8 exons that generate four alternatively spliced mRNAs all of which encode the same 487 amino acid protein. The HRH2 gene is located on chromosome 5q35.2 and is composed of 8 exons that generate two alternatively spliced mRNAs. HRH2 isoform 1 is 397 amino acids and HRH2 isoform 2 is 359 amino acids. The HRH3 gene is located on chromosome 20q13.33 and is composed of 4 exons that encode a 445 amino acid protein. The HRH4 gene is located on chromosome 18q11.2 and is composed of 5 exons that generate three alternatively spliced mRNAs. Isoform 1 is 390 amino acids. Isoform 2 is 302 amino acids. Isoform 3 is 67 amino acids.

The H1R protein is coupled to a Gq-type G-protein. These receptors are expressed in a wide variety of tissues including the gastrointestinal tract, central nervous system (specifically cells of the tubermamillary nucleus of the hypothalamus), both airway and vascular smooth muscle cells, endothelial cells, chondrocytes, monocytes, neutrophils, dendritic cells, T and B lymphocytes, adrenal medulla, and the cardiovascular and genitourinary systems. Expression of H1R in lymphocytes and monocytes is critical to the role of this receptor in mediating the modulatory effects of histamine within the immune system. With its distributed expression the H1R participates in ileum contraction,
modulates circadian cycle,
systemic vasodilatation,
bronchoconstriction (allergy-induced asthma), and mediates pruritus (sensation of itchy skin).

The H2R protein is coupled to a Gs-type G-protein. Most immune cells that express the HRH1 gene also express the HRH2 gene. In addition to immune cells, the HRH2 gene is expressed in gastric parietal cells where the receptor plays a critical role in the regulation of gastric acid production in response to histamine. Indeed, histamine is the major molecule responsible for increased gastric acid production. Acetylcholine (ACh) and gastrin also contribute to gastric acid production but not to the degree of histamine. The HRH2 gene is also expressed in additional cells of the gastrointestinal tract, cells of the central nervous system, smooth muscle cells, endothelial cells, and cardiomyocytes. The principal responses to histamine binding to the H2R are stimulation of gastric acid secretion,
increased sinus rhythm (cardiac rhythm initiated via depolarization from the sinus node), smooth muscle relaxation,
inhibition of antibody synthesis, inhibition of T-cell proliferation and inhibition of cytokine production.

The H3R protein is coupled to a Gi-type G-protein. Expression of the HRH3 gene occurs nearly exclusively within the central nervous system, CNS. Regions of the brain that contain cell types expressing H3R include basal ganglia, cortex, hippocampus and strial area. Within the CNS the effects of histamine binding to H3R include decreases in acetylcholine (ACh), serotonin and norepinephrine production and release. Outside the CNS low level HRH3 expression has been found in the gastrointestinal and cardiovascular systems and in bronchial cells of the lungs.

The H4R protein is coupled to a Gi-type G-protein. Expression of the HRH4 gene is seen at high levels in the gastrointestinal tract, spleen, thymus, medullary cells, bone marrow and peripheral haematopoietic cells, including eosinophils, basophils, mast cells, T-cells, leukocytes and dendritic cells. Lower levels of expression of HRH4 are detected in the brain, heart, liver and lung.